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| Home > Publications database > Synthesis of CMR manganites and ordering phenomena in complex transition metal oxides |
| Dissertation / PhD Thesis/Book | PreJuSER-62607 |
2008
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-89336-527-2
Please use a persistent id in citations: http://hdl.handle.net/2128/3141
Abstract: This dissertation covers: the instrumentation development; the optimization synthesis of polycrystalline materials; the single crystal growth of perovskite manganites; the determination of crystal and magnetic structures from x-ray and neutron powder diffraction data of polycrystalline and powdered single-crystalline CMR La$_{1-x}$Sr$_{x}$MnO$_{3}$ manganites as well as its application in quantitatively correlating structural and magnetic properties; the application of resonant soft x-ray scattering in the investigation of complex ordering phenomena in La$_{7/8}$Sr$_{1/8}$MnO$_{3}$ and La$_{1}$Sr$_{2}$Mn$_{2}$O$_{7}$ at the transition metal L$_{II, III}$-edges and the rare earth M$_{IV, V}$-edges. Two new devices have been developed: one is the rotating specimen holder for laboratory x-ray powder diffraction, which not only can effectively minimize the problem of preferred orientation but also produces a ‘zero-background’ contribution to the collected data; another is the miniature goniometer for resonant soft x-ray scattering, which makes it feasible to adjust samples along $\chi$ (±2.5°) and $\Psi$ (360°) and dramatically improves the efficiency of soft x-ray scattering chambers and can be utilized to unravel the electronic ordering phenomena by resonant soft x-ray scattering. The materials synthesis is the first and critical step for any research in condensed matter physics. One good route has been acquired for preparing high-quality polycrystalline La$_{7/8}$Sr$_{1/8}$MnO$_{3}$ samples, i.e. two times calcining and two times sintering, through exploring various preparation parameters. This synthesis route can be also used to prepare high-quality La$_{1}$Sr$_{2}$Mn$_{2}$O$_{7}$ and La$_{0.83}$Ca$_{0.17}$MnO$_{3}$. Thus it is a universal one for the present testing. The route for optimizing the synthesis of polycrystalline manganites has been established. One method for preparing straight and very uniformly densified feed rods with a homogenous composition distribution has been acquired, which makes growing large high-quality single crystals possible. Three nominal La$_{1-x}$Sr$_{x}$MnO$_{3}$ (x = 1/8) polycrystals prepared under different conditions air, Ar and O$_{2}$ at 1 atm show a huge contrast in the Curie temperature T$_{c}$. A combined Rietveld refinement of the room-temperature x-ray and neutron powder diffraction data has been performed for these three samples. This study not only can determine the relative content of the various elements (especially the oxygen) with higher precision as compared to one probe alone, but also makes it possible to correlate the drastic change in Tc with the detailed structural parameters. In addition, the individual effect of the number of nearest magnetic neighbors on the structural and magnetic properties has been investigated in detail using a set of La$_{7/8}$Sr$_{1/8}$Mn$_{y}$O$_{3}$ samples with initial y = 0.97, 1.00 and 1.03. Moreover, a systematic neutron powder diffraction study of the Ar annealed and air sintered La$_{7/8}$Sr$_{1/8}$Mn$_{1- \gamma}$O$_{3+ \delta}$ samples between 10 and 900 K has been performed, where the structural and magnetic phases and their respective transitions for two samples are completely different. For the air sintered sample, a structural transition from rhombohedral to orthorhombic upon cooling from 180 to 65 K has been observed for the first time. From the refined parameters the cooperative Jahn- Teller distortion parameter $\Delta$, the tolerance factor $\textit{t}$ based on the equilibrium La(Sr)-O and Mn-O bond lengths, and the one-electron bandwidth $\textit{W}$ of the e$_{g}$-band have been determined.
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